Project Abstract Group B streptococci (GBS) is a normal commensal bacterium found in the intestine and vagina of ~30% of healthy humans. Worldwide, GBS is the leading cause of pneumonia, septicemia and meningitis in the first 3 months of life and is the most important invasive pathogen in otherwise healthy newborn infants. While the prevalence of GBS-induced disease has been vastly diminished in neonates due to intrapartum antibiotics, the incidence, mortality, and morbidity of ?late? onset disease (7-90 days after birth) has remained unchanged over the last few decades. Furthermore, the disease remains one of the most common forms of sepsis and meningitis in the developing world where prepartum screening for vaginal GBS carriage is not routine and hence antibiotic prophylaxis is not offered to mothers. Defining how GBS activates the innate immune system to cause disease remains an important scientific and medical challenge. During the last decade, we reported on three distinct pathways responsible for the inflammatory response to GBS: a TLR2/MyD88 dependent pathway, an IFNalpha/beta pathway, and the NLRP3 inflammasome pathway. In addition, recent work from our group and others indicates that innate immunity against GBS depends on recognition of nucleic acids by an intracellular cytosolic DNA sensing pathway. DNA released into the cytosol during infection or introduced by transfection binds to and activates an enzyme cyclic- GMP-AMP synthase (cGAS). DNA-activated cGAS produces a cyclic di-nucleotide, 2'3'-cGAMP, which acts as a second messenger and triggers a protein stimulator of interferon genes (STING). STING, in turn, activates TANK-binding kinase 1 (TBK-1) leading to IRF3 activation and type I interferon (IFN) gene expression. Bacteria derived cyclic-di-nucleotides can also activate STING. In this proposal we will explore the role of bacterial nucleic acids in the activation of type I IFN responses during GBS infection. We hypothesize that GBS DNA activates cGAS, generating cGAMP and triggering STING. We also hypothesize that bacterial c-di-AMP, which we have shown is produced in abundance by GBS, activates STING. Surprisingly, and in contrast to what is known about other extracellular Gram-positive bacteria (e.g., pneumococcus), this type I IFN response to GBS is protective. Complicating this picture is that GBS encodes for two enzymes that metabolize cyclic-di-nucleotides. Using mutants in the major phosphodiesterase (CdnP), we have observed STING-dependent, cGAS-independent activation of the type I interferon response. We will use WT, STING KOs and cGAS KOs, in combination with related GBS mutants, to define the key pathways and molecular events that shape the innate immune response to infection. The outcome of these studies will guide the development of novel strategies and therapeutic interventions to ameliorate or prevent the damaging sequelae of GBS infection.